请问var result=“”；中为什么给result初始化为“”

function count(){
    var a = document.getElementById("txt1").value;
    var b = document.getElementById("txt2").value;
    var c= document.getElementById("select").value;
    var result ="" ;
    switch(c) {
        case "+":
            result = parseFloat(a) + parseFloat(b);
            break;
        case "-":
            result = parseFloat(a) - parseFloat(b);
            break;
        case "*":
            result = parseFloat(a) * parseFloat(b);
            break;
        default:
            result = a/b;
    }
    document.getElementById("fruit").value = result;
   
   }